1. Field of the Invention
The present invention relates to a solid-state image pickup device used widely in video cameras and the like.
2. Description of the Related Prior Art
The solid-state image pickup device is widely used in video cameras and the like. As the body of the video camera becomes smaller in size and lighter in weight, the solid-state image pickup device is required to have higher resolution and higher performance. Especially in the case of the frame interline transfer system used in a professional camera, it has been desired to decrease the occurrence of smear noise. It has been difficult to satisfy all these requirements by the mere extension of the prior art.
A conventional solid-state image pickup device its described below by reference to a drawing.
In FIG. 11, in the n type semiconductor substrate 1, the p type well 2 and the n.sup.- diffusion layer region 3 form a photoelectric converting part. When the light enters the n.sup.- type diffusion layer region 3 an electron and hole pair is generated. Electrons are accumulated in the n.sup.- type diffusion layer region 3. When very strong light enters, electrons overflow from the n.sup.- type diffusion layer region 3. Excess electrons flow into the n type semiconductor substrate 1 which is applied with a positive voltage. This structure is called a vertical overflow drain structure. The vertical shift register for transferring signal charges, the n type diffusion layer region 4 (hereinafter called vertical CCD register part), forms a buried channel type transistor structure. By forming the p type diffusion layer region 5 in the periphery thereof, invasion of electrons from the photoelectric converting part of the p type well 2 is prevented.
The p.sup.+ type diffusion layer region 6 can control the threshold value for transferring electrons from the photoelectric converting part to the vertical CCD register part. The p.sup.++ type diffusion layer region 7 forms a junction and separates adjacent pixels. In actual driving, for example, a high voltage of 10 V is applied to the polysilicon electrode 9, and all electrons accumulated during photoelectric converting are transferred to the vertical CCD register part. Consequently, for example, by applying 0 V and -10 V voltages alternately to the polysilicon electrode 9, the signal charges in the vertical CCD register part can be transferred sequentially.
The polysilicon electrode 9 is electrically insulated by the polysilicon oxide film 10. In its upper part, moreover, the interlayer dielectric film 11 of CVD film or the like is formed to insulate from the aluminum wiring. In the case of a solid-state image pickup device, the aluminum wiring also serves as the light-shield film. The aluminum film 12 is located in the upper part of the vertical CCD register part (polysilicon electrode 9), and prevents light from entering into the vertical CCD register part. Thus, in the conventional solid-state image pickup device, by forming the p type diffusion layer region 5 around the buried channel of the vertical CCD register part, invasion of electrons from the substrate inside is prevented. Besides, by forming the aluminum film 12 in the upper part of the polysilicon electrode 9, excess light is prevented to enter, thereby suppressing the occurrence of smear noise.
On the other hand, the contact window 14 is formed in a part of the polysilicon oxide film 10 and interlayer insulation film 11. Each polysilicon electrode 9 is lined by aluminum wiring. As a result, the driving speed of the solid-state image pickup device is enhanced. The driving pulse used in the solid-state image pickup device is applied from both ends of the polysilicon electrode 9 in the photoelectric converting part. Accordingly, in the middle of the photoelectric converting part, the waveform of a signal pulse is likely to deform. Hence, the transfer efficiency of a signal charge in that region is lowered. It is reported that the transfer efficiency of the signal charge is not lowered by improving the waveform of the driving pulses to be applied to the middle of the photoelectric converting part. Therefore, by lining with the aluminum wiring, deformation of capacity of the polysilicon electrode 9 can be suppressed, and as a result, the transfer efficiency can be enhanced.
In the prior art structure, however, in the bottom pare of the contact window 14 of the polysilicon electrode 9 and the aluminum wiring, the aluminum film having a high work function contacts with the polysilicon which has a low work function, so that a eutectic mixture is formed. At this time, the polysilicon electrode 9 is subjected to the effect of a high work function of the aluminum film, and its work function becomes high in its contact window 14.
In the solid-state image pickup device, herein, the vertical CCD register part of the buried channel structure is used. Hence, if the value of the work function of the polysilicon electrode 9 varies, the potential depth of the buried channel itself is changed. That is, when a eutectic mixture of polysilicon electrode 9 and aluminum wiring is formed, the potential depth of the buried channel formed beneath the contact window 14 becomes shallow. As a result, the transfer efficiency of signal charge may deteriorate, or the maximum amount of signal charge that can be transferred may decrease. In particular, when used in the environments of low illumination, it is hard to recognize the images of the solid-state image pickup device.
Besides lining with the aluminum film, it has also been considered to increase the doping dose of phosphorus in the polysilicon electrode a to lower the resistance to transfer at a higher speed. However, the polysilicon oxide film 10 is formed by oxidizing the polysilicon electrode 9 which has a low resistance. The polysilicon electrode 9 of low resistance has accelerated oxidation, thereby resulting in a greater film thickness of the polysilicon oxide film 10. More specifically, the polysilicon oxide film 10 is a second gate oxide film of the polysilicon electrode 9 of the first layer. Hence, the gap is widened between the polysilicon electrode 9 of the first layer and the polysilicon electrode 9 of the second layer because of the increased thickness of the polysilicon oxide film 10. When the gap is widened, the fringing electric field can hardly act in the CCD register part formed from the polysilicon electrode 9 of the first layer to beneath the polysilicon electrode 9 of the second layer. It leads, accordingly, to deterioration of transfer efficiency between the CCD register part formed beneath the polysilicon electrode 9 of the first layer and the CCD register part formed beneath the polysilicon electrode 9 of the second layer.
On the other hand, in order to avoid deterioration of the transfer efficiency the film thickness of the polysilicon oxide film 10 of the first layer, which becomes the second gate oxide film, can be reduced by oxidizing a polysilicon film having a higher phosphorus doping dose than in the prior art mentioned above However, the crystallinity of the oxide film formed by this accelerated oxidation is coarse, and the film quality is poor. Accordingly, such polysilicon oxide film 10 has a dielectric breakdown voltage of about 20 to 30% lower than the film formed without accelerated oxidation. Therefore, the polysilicon oxide film 10 format by accelerated oxidation which has a film thickness similar to the conventional polysilicon oxide film 10 has dielectric breakdown voltage which is deteriorated by about 20 to 30% and which causes the reliability of the solid-state image pickup device to be lowered. Hence, when it is desired to increase the speed of transfer by lowering the resistance of the polysilicon film, the transfer efficiency characteristic and dielectric breakdown voltage characteristic are in mutually contradictory relation. In other words, it not practicable to satisfy these two characteristics simultaneously.
Besides, as the solid-state image pickup device is further downsized, the size of the contact window 14 formed in the polysilicon electrode 9 and aluminum wiring is becoming smaller. Accordingly, when depositing the aluminum film in the contact window 14 by the sputtering method, the step coverage of the aluminum film is not favorable. As a result, a region of a thin film thickness of the aluminum wiring occurs in the step portion of the contact window 14. In such region of aluminum wiring, the light passes, and the effect as the light-shield film decreases. Thus, the lightshield performance of the aluminum wiring deteriorates and light gets into the polysilicon electrode 9 beneath the contact window 14 and in the buried channel beneath it, so that the smear noise characteristic is impaired.
The invention is intended to solve the above problems, and it is an object thereof to present a solid-stage image pickup device exhibiting a high transfer efficiency and an excellent smear noise characteristic even at a low illumination.